#include<stdio.h>
#include<math.h>
int main(){
	double a,b,c;
	scanf("%lf%lf%lf",&a,&b,&c);
	double p=(a+b+c)/2.0;
    printf("%.1lf\n",sqrt(p*(p-a)*(p-b)*(p-c)));
	return 0;
}
